This invention relates to semiconductor optical devices such as lasers and optical amplifiers.
Optical Networks are currently of great interest primarily due to their ability to carry a large amount of information. Particularly significant are Dense Wavelength Division Multiplexing (DWDM) Systems which carry several wavelengths in a single optical fiber. Important components of such systems are Erbium-Doped Fiber Amplifiers (EDFAs) and Raman fiber amplifiers, which allow long-haul transmission of the signal light without regeneration. These amplifiers operate by pumping of charge carriers in the fiber with pump lasers to cause amplification of the signal light. The amplification is a strong function of the pump power, particularly for systems carrying several channels. For example, in a typical 80-channel system, two 980 nm pump lasers and four 1480 nm pump lasers, each operating at 150 mW, are employed for an EDFA. In systems using Raman amplifiers, four or more pump lasers at multiple wavelengths are typically used.
It is desirable, therefore, to increase the power generated by a pump laser in order to simplify the system. This can be done, for example, by increasing the length of the laser chip, but this tends to increase internal optical losses. The width of the active region of the laser could also be increased, but this typically produces an unwanted additional transverse mode. Another possibility is to use a Master Oscillator Power Amplifier structure, but this introduces complexity in fabrication and difficulty in coupling the pump light into a single mode fiber. (See, e.g., Cho, et al, xe2x80x9c1.9-W Quasi-CW from a Near-Diffraction-Limited 1.55-micron InGaAsP-InP Tapered Laser,xe2x80x9d IEEE Photonics Technology Letters, vol. 10, No. 8, pp1091-10 93 August 1998)).
It is desirable, therefore, to increase the power of a laser emitting light in a single mode. It is also desirable to increase the power of semiconductor optical amplifiers.
The invention in one aspect is a semiconductor optical device which includes an active region with an active layer having a first index of refraction, and a blocking region having a second, lower index of refraction. A fundamental mode is generated in the active layer when a bias is applied to the device, the mode experiencing effective refractive indices in the active and blocking regions. A semiconductor layer having an index of refraction higher than the blocking region is formed over both the active and blocking regions so that the layer is in closer proximity to the active layer in areas not covered by the blocking region so as to decrease the difference between the effective refractive index of the active region and the effective refractive index of the blocking region.
In accordance with another aspect, the invention is apparatus which includes an optical amplifier, and a semiconductor pump laser coupled to the amplifier. The laser includes an active region with an active layer having a first index of refraction, and a blocking region having a second, lower index of refraction. A fundamental mode is generated in the active layer when a bias is applied to the device, the mode experiencing effective refractive indices in the active and blocking regions. A semiconductor layer having an index of refraction higher than the blocking region is formed over both the active and blocking regions so that the layer is in closer proximity to the active layer in areas not covered by the blocking region so as to decrease the difference between the effective refractive index of the active region and the effective refractive index of the blocking region.
In accordance with the farther aspect, the invention is a method of forming a semiconductor optical device including the steps of forming an active layer having a first index of refraction, and forming a blocking region having a second, lower index of refraction. A fundamental mode is generated in the active layer when a bias is applied to the device, the mode experiencing effective refractive indices in the active and blocking region so that a semiconductor layer having an index of refraction between that of the blocking region and the active layer is formed over both the active and blocking regions so that the layer is in closer proximity to the active layer in areas not covered by the blocking region so as to decrease the difference between the effective refractive index of the active region and the effective refractive index of the blocking region.